This study retrospectively investigated the portal vein images of patients who underwent routine examination for clinical reasons. Approval by the institutional ethics committee and written informed consent were waived.

This retrospective study included 50 patients (22 females and 28 males; age range, 18 ~ 84 years; mean age, 53.42 ± 13.21 years) in Group A who underwent routine abdominal-enhanced CT and 50 patients (22 females and 28 males; age range, 22 ~ 76 years; mean age, 52.48 ± 12.20 years) in Group B for abdominal CT angiography (CTA) scans with matched body mass index (BMI), weight, age, and sex from June to September 2021 in our hospital. The patient characteristics are listed in Table 1. The inclusion criteria were as follows: (1) age greater than 18 years, and (2) portal vein image complied with diagnosis requirements, as the subjective score was 3 or higher (Table 2). Those who had moderate to severe fatty liver disease (liver attenuation < 40 HU) and portal vein invasion were excluded [13].

All patients were examined with the helical mode of a 320-row CT scanner (Aquilion ONE GENESIS Edition; Canon Medical Systems, Japan). All CT examinations were acquired in the craniocaudal direction with the patient in the supine position and in inspiratory breath-hold. The scan parameters were as follows: 0.5 mm × 80 rows; 0.5 s rotation time; D-field of view (FOV), L400 mm; tube voltage, 120 kVp; automatic exposure control (SURE Exposure 3D, Canon) with noise index (SD = 8 HU for nonenhanced scan and portal venous phases) was used for tube current modulation. The scanning ranged from the top of the diaphragm to the level of the anterior superior iliac spine. Patients were injected with iodinated contrast material (Group A: 320 mg I/mL, 1.5 mL/kg; Group B: 370 mg I/mL, 1.5 mL/kg) through the median cubital vein using a double-head power injector (injection rate: 3.0 mL/s in Group A and 4.5 mL/s in Group B), followed by a 30 mL saline flush at the same injection rate. In Group A, the artery phase and portal venous phase (PVP) acquisitions were performed at 28 s and 50 s, respectively, after the initiation of contrast medium administration. Intelligent tracking starting with arterial scanning was applied in Group B. A region of interest (ROI) was placed on the aorta ventralis, and the scan was started immediately when the CT value of the ROI reached the threshold of 180 HU. PVP was obtained 45.5 s after intravenous injection of contrast medium. All transverse CT images were reconstructed using adaptive iterative dose reduction 3D (AIDR 3D) with FC18 kernel for 1 mm slice thickness and 0.8 mm interval.

Radiation dose in terms of volume CT dose index (CTDIvol) of PVP in the two groups was recorded for further statistical analysis. Since the scan range might be different between abdominal-enhanced CT scans and abdominal angiography scans in clinical routine, the dose length product (DLP) and corresponding effective dose were not included for comparison.

Images in Group A were postprocessed with CE-boost (SURESubtraction Iodine map, Canon Medical Systems, Japan) for further enhancement of the portal vein. Both subjective and objective assessments of the portal vein with different branches in three types of images (i.e., Group A with CE-boost and without CE-boost, Group B) were statistically analyzed.

Two experienced radiologists (8 and 10 years of abdominal diagnosis) who were unaware of the patient’s clinical information and image processing methods evaluated the image quality of the three types of images. Volume-rendered (VR), maximum intensity projection (MIP), and multiplanar reformations (MPR) were used for three-dimensional display of the vascular system. It was scored 1 to 5 according to the following four aspects: the number of segmental branches observed, the clarity of the vessel wall, and the artifact and noise level of the images [14] (Table 2).

Objective evaluation included CT value, noise index (i.e., the standard deviation (SD) of the CT value), the signal-to-noise ratio (SNR), and the contrast-to-noise ratio (CNR). The SNR and CNR for vessels were calculated using the following equations: SNR = (CTvalue)/SD and CNR = (CTvein-CTliver)/SDbackground [14, 15]. ROIs in the main portal vein (MPV), inferior vena cava (IVC), right portal vein (RPV), left portal vein (LPV), third-level branch of the RPV (RPV-3), and third-level branch of the LPV (LPV-3) were objectively evaluated. The ROIs were placed at the center of the veins (Fig. 1): the area of ROI in the MPV and IVC was approximately 20 mm2, the area of ROI in the LPV and RPV was approximately 10 mm2, the area of ROI in the LPV-3 and RPV-3 were approximately 5 mm2. The CT value and SD of the liver were calculated as the average ROI area of 50 mm2 of the left lobe and right lobe, and the ROI was kept away from vessels. The SD of the right erector spinae muscle (RES) with a circular ROI of 100 mm2 was regarded as the background SD.

ROI location illustration: a liver, b MPV and IVC, c LPV, d RPV, e the third-level branch of the LPV, f right erector spinae

Statistical analyses were performed using R software (version 3.6.1; http://www.R-project.org). Quantitative parameters were expressed as the mean values ± standard deviations (SD). The Shapiro–Wilk test was used to assess the normality of quantitative data. In the case of normality of data, one-way analysis of variance (ANOVA) was used to analyze the difference between multiple groups. Otherwise, Kruskal–Wallis one-way ANOVA was applied. Independent samples t test with Bonferroni correction was used for multiple pairwise comparison. Otherwise, the Mann–Whitney U test with Bonferroni correction was used. The kappa value of concordance was used to measure the degree of agreement between the two experienced radiologists. Agreement was determined as follows: no agreement (kappa < 0), slight agreement (0 < kappa ≤ 0.2), moderate agreement (0.4 < kappa ≤ 0.60), substantial agreement (0.6 < kappa ≤ 0.8), and almost perfect agreement (kappa > 0.8) [12, 16]. A p value of < 0.05 was considered statistically significant for all tests.